Recent genetic advances have driven significant progress in scientists' abilities to classify and map neuronal cell types within the brains of mode organisms like laboratory mice. To better delineate neuronal cell types in the human brain, however, it is critical to have a deeper understanding of the way that neuronal cell types evolve across mammals. As a first step toward achieving this understanding, corresponding neuronal cell types will be directly compared in two closely related mammalian species: mice and rats. By closely examining differences in the properties of these cells, including the genes they express, we hope to identify genomic elements that control the properties of neuronal cell types, and to infer properties of the corresponding cell types in the human brain. Improving the precision with which we can classify human neuronal cell types could have a transformative impact on our ability to understand pathological changes in neuropsychiatric disease.